A Massachusetts museum's decision to part with 40 artworks, including two by illustrator Norman Rockwell, has touched off a debate over whether it's ever ethical to sell pieces of the collection to pay the bills.

"One of the most fundamental and long-standing principles of the museum field is that a collection is held in the public trust and must not be treated as a disposable financial asset," the American Alliance of Museums and the Association of Art Museum Directors said in a joint statement. The sale would be an "irredeemable loss," they added.

Leslie Ferrin, who runs an area company that represents artists, started a Facebook page for members of the local art community opposed to the sale called "Save the art at the Berkshire Museum of Natural History and Art." Members of the group say they hope to convince the museum to change its mind.

"Selling gifts is against every moral and ethical standard" of running a museum, she said.

At auction, the pieces are likely going to be sold to private collectors, and the public will lose access, Ferrin said.

The sale is necessary to ensure the museum's very existence, executive director Van Shields said.

The money raised will help establish a $40m (£30.5m) endowment and pay for $20m in renovations as the museum refocuses its mission to become a more interdisciplinary and interactive institution more dedicated to history and science.

"We are facing an existential threat. We needed to adapt, migrate or go extinct," he said.

The art being auctioned includes works by Albert Bierstadt, Alexander Calder and Charles Wilson Peale, but it's the Rockwell oil paintings that have stirred the deepest emotions.

"Blacksmith's Boy —Heel and Toe" and "Shuffleton's Barbershop" were gifts to the museum from Rockwell himself, who called the region home for the last 25 years of his life.

Laurie Norton Moffatt, director of the nearby Norman Rockwell Museum, has come out in opposition to the sale.

"For the museum's leadership, the potential price that these irreplaceable artistic treasures could fetch seems to have obscured their very rich role in the life of the Berkshires," she wrote in an opinion piece in The Berkshire Eagle newspaper.

Shields respects the opinions of those opposed to the sale, and said he and the museum's trustees "knew we were going to be pilloried," but added that the auction is a done deal.

When the Berkshire Museum opened in 1903, it was the cultural beacon of the region. It was founded by Zenas Crane, a member of the family that owned Crane & Company, a paper manufacturer that to this day supplies paper used to make U.S. currency.

Now it's overshadowed by the region's world-renowned museums, including the Clark Art Institute, the Massachusetts Museum of Contemporary Art and the Norman Rockwell Museum.

"The cultural landscape has changed dramatically and the Berkshire Museum has not adapted to that change," Shields said.

Selling the works was not a decision made lightly. It was a two-year process that involved multiple focus groups, multiple retreats by the board of trustees (which includes a member of the Crane family) and input from hundreds of members of the community, Shields said.

By switching its focus to science and history — and yes, some art will remain — the Berkshire Museum can fill a currently empty cultural niche in the region.